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Periodic Trends and Properties

Valence Shell and Valence Electrons
The outermost shell of an atom is known as the valence shell, and the number of electrons present in the valence shell of an atom are known as the valence electrons.
Groups of Elements in a Periodic Table

Significance of valence electrons

  1. The valence electrons of an atom are responsible for the chemical reactivity and these electrons take part in a chemical bonding.
  2. The valence electrons in an atom determine the mode of chemical combination, i.e. whether the atoms would combine to form ionic or covalent bond (described in the chapter on ‘Chemical Bonding’).
  3. The valence electrons determine the combining capacity or the valency of an atom. The number of electrons in an atom that actually take part in bond formation is known as the valency of that element.
    For example, C has four valence electrons. The C atom is capable of forming four bonds. Hence, the valency of C is 4.
  4. If the outermost shell of an atom is completely filled, then its valency is zero.
    For example, the outermost shells of neon, argon etc. are completely filled. Hence, the valency of these elements is zero.
  5. Elements having the same number of valence electrons in their atoms possess similar chemical properties.
    For example, all alkali metals have one valence electron in their atoms. Hence, their chemical properties are similar.
  6. Elements having different number of valence electrons in their atoms show different chemical properties.
    For example, all alkali metals have one valence electron in their atoms, whereas halogen atoms have seven valence electrons. Their chemical properties are entirely different from each other.
  7. The number of the valence shell in the atom of an element determines the period number of the element in the periodic table.
    For example, the valence shell number of sodium (Na) is 3. Hence, sodium is placed in period 3 of the periodic table.
  8. Elements with 1, 2 or 3 valence electrons in their atoms are metals. The exceptions are hydrogen and helium.
  9. Elements with 4, 5, 6 or 7 valence electrons in their atoms are non-metals.
Valency: Valency of elements in a period generally increases from one to four and then decreases to zero. It is zero for noble gases.
Atomic radius (atomic size): The distance between the nucleus of an atom and its outermost shell having electrons is known as its atomic radius.
Ionisation energy (IE): The minimum energy required to remove the outermost electron from an isolated gaseous atom is called the ionization energy.
Electron affinity: It is the amount of energy released when an electron is added to an isolated gaseous atom.
Electronegativity: The tendency of an atom in a molecule to attract a shared (bonded) pair of electrons towards itself is termed as the electronegativity of the atom.

Metallic and Non-metallic Properties

In the periodic table, elements to the left of the zig-zag line shown (boron to astatine) are metals and those to the right are non-metals. The bordering elements, boron (B), silicon (Si), germanium (Ge), arsenic (As), tellurium (Te) and polonium (Po) show properties of metals as well as non-metals. These are called metalloids. Properties of metals and non-metals are shown in Table.

Properties of Metals and Non-metals
Metals Non-metals
Possess metallic lustre Do not possess lustre
Are hard crystalline solids Exist as soft solids, liquids and gases
Have high densities Have low densities
Have high melting and boiling points Usually have low melting and boiling points
Are malleable and ductile Are brittle, non-malleable and non-ductile
Good conductors of heat and electricity Bad conductors of heat and electricity
Are sonorous (except Na, K, Hg) Are non-sonorous
Form alloys with the metals and non-metals Do not form alloys with the other non-metals


Metallic and Non-metallic Characters of Elements with Reference to the Periodic Table
  1. A metal is characterized by 1, 2 or 3 electrons in the outer shell.
  2. Metallic character decreases across a period and increases down a group.
  3. As metals have a tendency to lose electrons, they are basic in nature and act as reducing agents.
  4. Metals combine with oxygen to form basic oxides.
  5. Non-metals generally possess four to eight electrons in the outermost shell.
  6. Non-metallic character increases across a period and decreases down a group.
  7. As non-metals have a tendency to accept electrons, they are acidic in nature and act as oxidizing agents.
  8. Non-metals combine with oxygen to form acidic oxides.
Acidic oxides on treatment with water form acids while basic oxides form bases.

General Metallurgy

The metallic compounds occurring in earth’s crust are called minerals, and the earthly impurities present along with these minerals are known as gangue or matrix. Some of the minerals are provided in Table.
The mineral from which a metal can be extracted conveniently and economically is called an ore.
General principles of metallurgy The process of extracting the metal from the ore, refining the crude metal and the manufacture of alloys from metals is called metallurgy. The stages or steps involved in metallurgy are as follows:


  1. Atomic radius: Decreases across the period and increases down the group.
  2. Ionisation energy, electron gain enthalpy, electronegativity increase across the period and decrease down the group.
Figure 3.3 Variation of Periodic Properties Across the Period and Down the Group

Important Minerals of Metals


Names of the Ores










Al2O3 · 2H2O














Copper glance






Iron pyrites






Zinc blende






Copper pyrites










CaCO3 · MgCO3



CuCO3 · Cu(OH)2



CaSO4 · 2H2O


Epsom salt

MgSO4 · 7H2O






KCl · MgCl2 · 6H2O


Rock salt



Horn silver








Native ore

Au, Ag, Cu, Os, Ir, Rh, Ru, Pd, Pt, As, Sb, Bi

All ores are minerals, but all minerals are not ores.

  1. Crushing or grinding of the ore: In nature, ores occur as huge lumps. They are broken down into small pieces and reduced to a fine powder. This process is called pulverisation.
  2. Concentration of the ore: The process of removal of impurities (gangue) from the ore and thus increasing the percentage of ore particles is called concentration and the purified ore is called the concentrated ore. It is also known as ore dressing or ore benefaction.
    1. Gravity separation or levigation or hydraulic washing: Gravity separation is employed when the ore particles are heavier than the gangue particles.
    2. Magnetic separation: Magnetic separation is suitable for ores which are magnetic in nature or contain magnetic impurities.
    3. Froth-flotation: Froth-flotation process is used for concentrating sulphide ores.
    4. Chemical process: Chemical process is used to separate the impurities by dissolving the ore in a suitable reagent.

Figure 3.4 shows the classification of metallurgy.


Chemical processes

  1. Conversion of the ore into its oxide:
    1. Calcination: It is the process of heating the ore to a high temperature below its melting point either in a limited supply of air or in the absence of air to bring about the decomposition and to remove volatile impurities.
      For example, conversion of carbonates and hydrated oxides into their respective oxides.
    2. Roasting: It is the process of heating the ore to a high temperature below its melting point in the presence of excess of air.
      For example, conversion of sulphide ores into their respective oxides.
  2. Reduction of oxide into metal:
    1. Pyrometallurgy: It is the reduction process carried out at high temperatures. Here, ore is reduced to metal using a reducing agent.
      For example, ZnO + C → Zn + CO.
    2. Hydrometallurgy: It involves the dissolution of the ore in a suitable chemical reagent and then precipitation of the metal by a more electropositive metal.
      For example, extraction of silver and gold.
    3. Electrometallurgy: It is the process of reduction of the metal halide or oxide by using electrical energy.
      For example, electropositive metals (Na, K, Al etc.) are extracted by this method.
  3. Refining of metals: The process of purifying the crude metal is known as refining.
    1. Oxidation: It is especially used for the purification of iron which contains impurities of carbon, silicon and sulphur.
    2. Liquation: It is used for metals having low melting points such as tin and lead.
    3. Poling: In this process the metal is stirred with wooden poles which reduce the oxide to metal. Copper metal is purified by poling.
    4. Distillation: It is used to purify zinc and mercury.
    5. Electrolytic refining: It is an extensively used method for purifying many metals such as Cu, Ag, Au, Pt, Cr, Ni, etc.

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